In the content editor, hover over the form module and click the Edit styles icon.

In the form module editor that slides in on the left, you'll see various options for styling your form - font type, font colors, margins, border colors, etc. Make your desired changes and click Save and go to module list when you're done.

Click Publish or Update to take your changes live.

Please note: embedded forms on non-HubSpot pages need to be styled with CSS on the external site and/or page(s) they are embedded on.